Relocating speedo gear drive

Any one ever try to relocate the speedo gear drive to the rear axle? Seems like you would need to source a gear drive from a bike with a front tire about the same diameter as your rear and with a hole the same size as your axle. I've seen it done with Harleys and a SV650.

Most tyres have fairly similar circumference - what sizes do you use today?

Note that you will need something in the hub to drive the gears as well as the drive unit itself. On my Z650B1, the front and rear axels are the same diameter IIRC.